Biomass is biological material, such as plants or plant-derived materials. Biomass is a renewable energy source when burned to produce heat, or converted to various forms of bio-fuel. The thermal method to generate energy or electricity from biomass usually involves a stoker boiler with a furnace for burning the biomass that is fed into it. For many years, since the first biomass boilers where designed and manufactured, biomass was seen as a waste material that needed to be incinerated. During the last 20 years, with the escalating cost of fuels used to generate electricity, a new vision of biomass as a renewable fuel is changing the design conception of these boilers. Higher thermal efficiencies with lower particulate emissions are driving many boiler design changes. Controlled biomass deposition on the furnace grate using improved air spreading systems is one of the major goals encountered in the new designs. Trying to avoid biomass piling on the grate, many boilers are operated with excess air as well as high carryover of unburned particulate.
Some studies on sugar cane bagasse fired boilers have found that maintaining a uniform thin bed of bagasse, between 1″ and 3″ inches (25 to 75 mm) deep, over the complete area of the grate, assures a continuously burning grate bed which rapidly dries and heats the bagasse fibers in suspension, acting as pilot flames for the incoming fuel stream. When the bed is partially uncovered or has very thin beds, less than 1″ inch deep (about 25 mm), the ignition zone, immediately above contains an unstable and highly fluctuating flame of low luminosity that induces combustion cycling which becomes evident with furnace puffing or cycled pressurization. When the bagasse accumulates in piles above 6″ (meaning six inches) deep, it reduces the grate heat release. Accordingly, optimizing partial biomass distribution on the grate, while burning the rest in suspension, with minimum excess air, is ideal for stable combustion and efficient steam generation.
FIG. 1 depicts a prior art biomass spreading system 120, coupled to a furnace 104 of a typical boiler 100 including a grate 106. The grate 106 can be fixed or travelling in a horizontal or inclined fashion. The grate 106 illustrated in FIG. 1 is a horizontal stationary pinhole grate. Various Biomass distributors 108 are attached to a front wall 109 of the furnace 104. Through the biomass distributors 108, biomass material 132 is fed into the furnace 104. Under grate air 142 is fed into a furnace chamber 133 by a forced fan 135. Air passes through many small holes on the grate 106 to provide oxygen for burning the biomass material 132. To distribute the biomass material 132 over the furnace grate 106, a biomass distribution system 120 is operatively coupled to the biomass distributors 108.
FIG. 2 presents a zoomed view of FIG. 1, and details of the biomass distribution system 120 that is operatively coupled to the distributor 108. The biomass material 132 is spread into the furnace 104 by the sweeping action of air passing through a narrowly slotted passage 131 which is a part of the biomass distributor 108. The air is supplied by the fan 110. The distribution system 120 includes a main header 122, which feeds various secondary ducts 123 that in turn feed various valve housings 124. Each valve housing 124 contains one or two dampers. One of the dampers is a rotary damper 126, while the other, if it exists, is a manual damper 127. As air flows from the valve housing inlet 121 through the passages left open by the dampers 126 and 127, it loses pressure depending on the variable open area of these passages. The valve housing outlet 150 discharges into a header 151 after a 90° (meaning 90 degrees) air flow turn from the valve housing 124. Another 90° flow turn is required to exit the header 151 and enter a rectangular duct 152 which connects to the distributor 108 with a flange 153.
The sudden changes in direction of the air flow as well as the sudden contractions described above create high turbulence and high pressure drops, and thereby reducing the effectiveness of the air jet 130 in sweeping the biomass material 132 into the boiler 100. An electric motor (not shown) provides rotation to a shaft 125, common to all the rotary dampers 126, inside the valve housings 124. The valve housings 124 feed sweeping air to all the biomass distributors 108 in a stoker boiler. The rotary damper blade 126 of each valve housing 124 is set in a position different from the rest, so that they will create different pressure drops as the blades 126 rotate simultaneously. In other words, when one damper 126 is in the open position, the other dampers 126 are closed to various degrees. Accordingly, each blade 126 is at a different rotation position from the other blades 126. The manual dampers 127 are set individually, based on the boiler operators' experience, to establish a minimum sweeping flow to help distribute the biomass evenly over the grate 104.
When any rotary damper 126 is at the closed position, it partially or substantially blocks the air flow from the secondary duct 123 to the discharge duct 152. In such a case, the biomass distribution system 120 provides the lowest air pressure in the discharge duct 152, minimizing the air sweeping action for biomass spreading. After the rotary damper 126 rotates 90° from the closed position, it is in the open position. At the open position, the rotary valve 126 provides the least resistance to the air flow from the secondary duct 123 to the discharge duct 152. In other words, when the rotary valve 126 is at the open position, the biomass distribution system 120 provides the highest air pressure in the discharge duct 152, maximizing the air sweeping action for biomass spreading.
Air flows from the discharge duct 152 into distributor 108 and through the air sweeping nozzle 131, thereby creating the air jet 130. The biomass material 132 is fed vertically down into the distributor 108 by a biomass feeder (not shown). The air jet 130 velocity (meaning the velocity of the air jet 130) is the result of the air flow contraction as it passes through the air sweeping nozzle 131, and encounters the biomass material 132 falling through the distributor 108. The air jet 130 momentum (meaning air mass multiplied by air velocity of the air jet 130), created by the air jet 130 passing through the air sweeping nozzle 131, pushes the biomass 132 into the furnace 104. When the air pressure in the discharge duct 152 is at the highest point, the air jet momentum is expected to be the highest level and the biomass material 132 moves furthest into the furnace 104. In such a case, the biomass material 132 falls onto an area of the grate 106 that is close to a back wall 107 (see FIG. 1) of the furnace 104. In contrast, when the air pressure in the discharge duct 152 is at the lowest level, the biomass material 132 travels a shortest distance into the furnace 104 and falls on the area of the grate 106 that is closest to the front wall 109.
Even distribution of the biomass material 132 over the grate 106 is very important for the reasons described above and other reasons described below. For example, an even distribution allows for higher biomass burning capacities as well as higher and more stable heat release rates, which in turn provide higher boiler steam generation at stable pressure and temperature. As an additional example, the thermal efficiency of a biomass stoker boiler is reduced when the biomass covers the grate unevenly, meaning that some areas have a thick bed while other areas have a thin bed. The uneven distribution of biomass 132 on the grate 106 forces the operators to work with more excess air, an unnecessarily high quantity of unburned fibers and incombustibles carried over by the flue gases.
Accordingly, the prior art biomass distribution system 120 fails to spread the biomass material 132 evenly over the furnace grate 106. The main reason for the failure is that the system 120 cannot control the momentum variation of the air jet flow 130, with respect to time or observed biomass bed deposition depth over the grate 106. Such limitation of the system 120 is caused by a number of reasons. First, the system 120 does not provide a controlled air jet 130 momentum variation with respect to time, because it does not provide a controlled variation of pressure behind the air sweeping nozzle 131 during the damper rotating cycle. Second, the system 120 does not allow for individual adjustment of air pressure to a distributor 108 independently from the other distributors 108, because the system 120 is operated by a single motor through a common shaft. Third, the system 120 creates high air pressure losses and turbulence that reduce the sweeping effectiveness of the air jet 130, thereby requiring higher fan pressures and causing higher energy cost and less sweeping control.
FIG. 3 illustrates a graph depicting the typical air pressure behind the prior art air sweeping nozzle 131 (10 to 20 inches of water column (“inWC”)) during a cycle of ten (10) seconds corresponding to a 90° rotation of the damper 126. As shown by the graph, during the latter 35% of the cycle (about three and a half seconds), the air pressure behind the sweeping nozzle 131 stays almost constant at 18 inWC. Beyond the first six seconds of the cycle, the air pressure decays almost linearly from 17 to 7 inWC. Accordingly, the graph indicates that most of the biomass 132 is spread towards the rear zone of the furnace grate 106. In other words, piles of the biomass 132 are formed in the rear zone of the grate 106 and are not burned efficiently. In contrast, the section of the grate 106 near the front wall 109 tends to remain uncovered, thereby lowering heat release rates. In fact most prior art biomass boilers depend on frequent manual spreading of the piled biomass in order to maintain desired steam production levels. The manual spreading is accomplished by opening manhole doors (not shown) located at the front wall 109 and below the distributor 108 openings, manually introducing long spreading rakes, and dragging the piled biomass so as to spread it evenly over the depth and width of the grate 106.
To correct the uneven distribution of the biomass material 132 over the grate 106, operators of the system 120 usually try to throttle the air pressure. However, the reduction in the air pressure fails to solve the problem of uneven distribution of the biomass material 132 over the grate 106. Rather, the reduction in the air pressure shifts the uneven deposition of the biomass 132 towards the front section of the grate 106. In addition to the problem of uneven distribution along the depth of furnace grate 106, there is the problem of uneven distribution across the width of the furnace grate 106 due to variations in feeder discharge. The system 120 does not allow individual adjustments of each air jet 130 to each distributor 108 over the complete cycle, it can only effect de minimis adjustments in air flow passing through the manually adjustable damper 127.
Neither does the prior art system 120 allow for individual adjustments to each jet flow 130 in response to higher bagasse density and/or friction as it moves through the distributor 108. Higher bagasse density is caused by, for example, higher moisture content. Another disadvantage of the prior art system 120 is that it creates very high turbulence and pressure losses for numerous reasons, such as inefficient flow throttling through single blade butterfly dampers, sudden changes in direction and flow contractions as air flows through the valve housing 124 and into the lateral exit port 150, and sudden change in flow direction as air flows out of the header 151 into the lateral rectangular duct 152. The air flow is highly irregular and thus creates high turbulence when it exits the duct 152. The momentum of air jet 130 is thus reduced. In other words, the current state of the art distribution system 120 fails to provide even biomass distribution. Such shortcomings of the prior art system become even worse when there is higher moisture content or uneven biomass feeding from one feeder to another. Furthermore, the system 120 consumes more fan power than necessary.
Accordingly, there is a need for a new biomass distribution system that evenly distributes biomass over a grate surface.